Field
The present invention relates to a high electron mobility transistor manufacturing method and a high electron mobility transistor manufactured by the manufacturing method.
Background Art
Semiconductor elements formed by using nitride semiconductors are promising high-voltage or high-speed elements because of inherent characteristics of compound semiconductor materials. In recent years, high electron mobility transistors (HEMTs) formed by using nitride semiconductors have been gradually brought into practical use.
In conventional high electron mobility transistors formed by using GaN compound semiconductors, which are nitride semiconductors, a lower buffer layer of AlN or GaN is formed on a sapphire, Si, or SiC substrate at a low temperature, and a buffer layer of GaN and an electron supply layer of AlGaN are stacked thereon in this order, thus forming a heterojunction structure. A source electrode, a gate electrode, and a drain electrode are provided on the electron supply layer.
In such a high electron mobility transistor, two-dimensional electron gas formed directly under the heterojunction interface between the buffer layer and the electron supply layer is used as carriers. When bias voltages are applied to the source electrode and the drain electrode, electrons move through the two-dimensional electron gas layer at high speed to travel from the source electrode to the drain electrode. At this time, the current between the source electrode and the drain electrode can be controlled by controlling the voltage applied to the gate electrode to change the thickness of a depletion layer directly under the gate electrode.
To reduce a leakage current in the butler layer, the resistance of the buffer layer needs to be increased. In the case where the resistance of the buffer layer is not increased, a drain leakage increases which flows between the source and the drain through the buffer layer in a state in which the drain current is turned off. Japanese Patent Application Publication Nos. 2002-57158, 2003-197643, and 2007-251144 propose methods for increasing the resistance of a buffer layer. In the methods of Japanese Patent Application Publication Nos. 2002-57158, 2003-197643, and 2007-251144, a buffer layer made of GaN is doped with impurities such as Zn, Mg, or carbon to increase the resistance of the buffer layer.
The resistance of a semiconductor can also be increased by radioactive ray irradiation such as electron-beam or proton irradiation. Japanese Patent Application Publication No. S50-126180 and International Patent Application Publication No. WO2012/053081 disclose methods for increasing the resistance of a semiconductor by irradiating the semiconductor with an electron beam or protons.
To reduce the drain leakage, impurities need to be added to the buffer layer. However, if too many impurities are added to the buffer layer, turn-on characteristics are changed. For example, in high electron mobility transistors using GaN semiconductors, if too many impurities are added to a GaN semiconductor, traps formed by the impurities cause fluctuations in characteristics such as current collapse or threshold shifts. Current collapse is a phenomenon in which temporal changes in an output current lead to poor reproducibility of output current characteristics. It is inferred that such a phenomenon arises as follows: when a current is passed through a semiconductor element, some of added impurities are charged, and an electric charge built directly or indirectly influences electron motion in a two-dimensional electron gas layer.
Accordingly, it is difficult to achieve both of a reduction in fluctuations in turn-on characteristics and a reduction in drain leakage. High electron mobility transistors have been required in which drain leakage can be reduced by doping a butler layer with impurities while fluctuations in turn-on characteristics are reduced.